The structure and operation of moving coil loudspeaker drive units is well known. A vibration diaphragm is attached to a coil of wire known as a voice coil, and the voice coil is placed in a magnetic field usually provided by one or more permanent magnets. By passing an alternating current through the voice coil, a force is induced and the diaphragm can be made to vibrate and so radiate acoustic waves.
What is sometimes not appreciated is that the force induced in the voice coil also gives rise to an unintentional reactive force on the motor system, following Newton's third law of motion. The mechanical vibration resulting from the reactive force on the motor is transmitted via the driver chassis and can excite the walls of a loudspeaker enclosure; in many loudspeaker systems this form of excitation is the major cause of motion in the enclosure walls. Since the walls have large area and exhibit structural resonances they can radiate significant sound resulting in a tonally distorted output from the loudspeaker.
Various solutions have been proposed to avoid this magnet vibration. U.S. Pat. No. 4,805,221 is one of several which disclose a loudspeaker with two substantially identical diaphragms and drive assemblies, mounted back to back. The permanent magnets of each assembly are rigidly coupled together by tie bars such that any reactive force in one magnet is cancelled by the opposing reactive force in the other. In this way magnet vibration is reduced along with the corresponding sound radiation from the enclosure walls. UK patent application no 0411566.3 (publication no 2414620) discloses a development of this design, in which the tie bar has an adjustable length.
The present inventors have carried out laser vibrometry measurements of a loudspeaker according to the design of U.S. Pat. No. 4,805,221, and found that at frequencies of above a few hundred Hertz the forces on the drivers do not cancel due to the excitation of structural modes of resonance in the tie bars. An alternative solution is therefore required.